Electrochemical devices, including proton exchange membrane fuel cells, electrolyzers, chlor-alkali separation membranes, and the like, have been constructed from membrane electrode assemblies (MEAs). Such MEAs comprise one or more electrode portions, which include a catalytic electrode material such as Pt or Pd, in contact with an ion conductive membrane. Ion conductive membranes (ICMs) are used in electrochemical cells as solid electrolytes. In a typical electrochemical cell, an ICM is in contact with cathode and anode electrodes, and transports ions such as protons that are formed at the anode to the cathode, allowing a current of electrons to flow in an external circuit connecting the electrodes.
MEAs are used in hydrogen/oxygen fuel cells. A typical MEA for use in a hydrogen/oxygen fuel cell might comprise a first Pt electrode portion, an ICM comprising a proton-exchange electrolyte, and a second Pt electrode portion. Such an MEA can be used to generate electricity by oxidation of hydrogen gas, as illustrated in the following reactions: ##EQU1##
In a typical hydrogen/oxygen fuel cell, the ions to be conducted by the membrane are protons. Importantly, ICMs do not conduct electrons/electricity, since this would render the fuel cell useless, and they must be essentially impermeable to fuel gasses, such as hydrogen and oxygen. Any leakage of the gasses employed in the reaction across the MEA results in waste of the reactants and inefficiency of the cell. For that reason, the ion exchange membrane must have low or no permeability to the gasses employed in the reaction.
ICMs also find use in chlor-alkali cells wherein brine mixtures are separated to form chlorine gas and sodium hydroxide. The membrane selectively transports sodium ions while rejecting chloride ions. ICMs also can be useful for applications such as diffusion dialysis, electrodialysis, and pervaporization and vapor permeation separations. While most ICMs transport cations or protons, membranes that are transportive to anions such as OH.sup.- are known and commercially available.
Commercially-available ICMs are not entirely satisfactory in meeting the performance demands of fuel cells. For example, Nafion.TM. membranes (DuPont Chemicals, Inc., Wilmington, Del.) which are perfluorocarbon materials having a SO.sub.3.sup.-, anion, are inherently weak. Nafion.TM. membranes are not generally available at thicknesses of less than 50 .mu.m. One reason is that Nafion.TM. membranes that thin would require reinforcement, thus defeating the purpose of a thin membrane by increasing the overall thickness as well as increasing the electrical resistance of the membrane. While Nafion.TM. membranes with lower equivalent weight can be used to obtain lower electrical resistance, lower equivalent weight membranes are structurally weaker and still would not obviate the need for reinforcement.
One means of constructing a reinforced membrane is to imbibe or infuse an ion-conductive material into a porous inert reinforcing membrane to make a composite membrane. For example, Gore-Select.TM. membranes (W. L. Gore & Associates, Inc., Elkton, Md.) comprise a poly(tetrafluoroethylene) (PTFE) membrane having an ion-conductive or ion exchange liquid impregnated therein. U.S. Pat. No. 5,547,551 describes a PTFE membrane fully impregnated with Nafion.TM. solution for use in fuel cells. Other inert membranes have been mentioned, such as polyolefins and poly(vinylidene fluoride), as suitable carriers for ion-conducting electrolytes.
Composite proton exchange membranes, comprising electrolytes immobilized in porous webs, have been shown to offer superior properties over single component membranes when used in fuel cells. The composite membranes can be made thinner and stronger while giving equivalent conductivity with less electrolyte, and have more dimensional stability even after becoming saturated with water. However, because the membranes employed arc initially porous, the gas permeability of the resulting membrane depends in part on the degree to which the membrane is filled by the electrolyte.
These composite membranes are used in fuel cell MEAs that use conventional catalyst electrodes in the form of applied dispersions of either Pt fines or carbon supported Pt catalysts. These conventional catalysts are applied as a coating of ink or paste to either the composite membrane or to an electrode backing layer placed adjacent to the membrane. The ink or paste typically contains electrolyte in the form of an ionomer.
Various structures and means have been used to apply or otherwise bring a catalyst in contact with an electrolyte to form electrodes, e.g., cathodes and anodes. These "membrane electrode assemblies" (MEAs) can include: (a) porous metal films or planar distributions of metal particles or carbon supported catalyst powders deposited on the surface of the ICM; (b) metal grids or meshes deposited on or imbedded in the ICM; or (c) catalytically active nanostructured composite elements embedded in the, surface of the ICM.
Nanostructured composite articles have been disclosed. See, for example, U.S. Pat. Nos. 4,812,352, 5,039,561, 5,176,786, 5,336,558, 5,338,430, and 5,238,729. U.S. Pat. No. 5,338,430 discloses that nanostructured electrodes embedded in solid polymer electrolyte offer superior properties over conventional electrodes employing metal fines or carbon supported metal catalysts, including: protection of the embedded electrode material, more efficient use of the electrode material, and enhanced catalytic activity.